Although pain is a crucially important physiological response, it also results in unnecessary suffering and agony. The control and relief of pain is an important branch of medicine. Pain may come about both as a result of disease as well as a result of medical treatment such as chemotherapy. In either case, it is important to alleviate the pain as much as possible so as to enable the sufferer to function normally.
Two neural pathways relating to pain act concurrently in the body: (1) a sensory pathway which senses tissue damage and subsequently produces a feeling of pain; (2) an analgesic pathway which reduces the feeling of pain and prevents the flow of information about the pain to the central nervous system (CNS), thus allowing the organism to maintain it's normal activity in spite of an injury. Anesthesia can be realized either by use of a drug which inhibits peripheral nerves that act as pain sensors or by enhancement of the natural analgesic system. Since these are different pathways, they are affected by different substances. For example, aspirin and lidocaine are active on the peripheral sensory pathway, while morphine and related substances are active on the analgesic system.
The most efficient analgesics currently in use are morphine-related substances of opiatic origin. It's well known that the brain makes a variety of endogenic opiates, and this explains the powerful effect of these substances. Their action on neurons is mediated by specialized receptors. Signals regulated by these receptors prevent the flow of information from the peripheral pain neurons to the CNS. These CNS neurons are also sensitive to a variety of other chemical substances including catecholamines (serotonin, noradrenalin etc.), neuroactive peptides (neurotensin) and inhibitory amino acids (glycin and GABA).
Out of some 4000 currently living species of snakes, approximately 400 species are known to be venomous. The venomous species are classified into five families, being Viperidae, Elapidae, Crotolidae, Hydrophidae and Atractaspidae. Snakes of the Viperidae family are distributed in Europe, Asia and Africa, and comprise 8 genera, one of which is the genus Vipera The Crotolidae family includes the genus Crotalus. The Elapidae family includes the genus Naja. 
Snake venom comprises a large variety of different substances. Out of several hundreds of estimated compounds, it is believed that only 4–8 are involved in the toxic effect of the venom. Despite functional similarity, snake venoms differ considerably in their chemical composition. Each species possesses its own characteristic venom composition. To date, only a few hundred compounds from some 400 venomous snake species have been characterized. These include enzymes, toxins, growth factors, etc. Most of the isolated venom compounds are of unknown function.
Traditionally, snake venom is considered a source of toxic substances. However, it is also a source of analgesics. Doctors who treated patients bitten by a South American snake (Crotalus durissus terrificus) reported that although these patients were in a life-threatening condition, they felt no pain. A neurotoxin product isolated from snake venom was regarded as a new type of analgesic at the First Congress of Neurotoxicology (1977) in Yugoslavia. These and other observations led to attempts to isolate anesthetic compounds from snake venom.
Bevan, P. and Hiestand, P. (1983) J Biol Chem. 258:5319–5326 describe a single chain polypeptide isolated from Vipera russelli russelli venom by cation exchange chromatography. The polypeptide competes with the binding of monoamines and opiate ligands to their respective receptors, and injection of the polypeptide intracerebroventricularly in rats causes marked sedation. The authors state that the polypeptide is a large and highly charged molecule which is unlikely to pass the blood-brain barrier. The polypeptide was found to be a moderately potent toxin, similar to the crude venom.
Dutta, A. S. and Chaudhuri, A. K. N. (1991) Indian J Exp. Biol 29:937–942 describe experiments carried out with crude venom of Vipera ruselli on mice and rats. The venom was injected intraperitoneally and intravenously, and was found to produce alterations in general behavior patterns connected with the CNS The venom showed significant analgesic activity in one assay, but no activity in two other assays.
WO 91/01740 published Feb. 21, 1991 discloses the use of lyophilized Crotalus atrox whole venom in a pharmaceutical composition for external use. The composition has analgesic, hyperaemizating and spasmolysant activity.
Giorgi, R., Bernardi, M. M. and Cury, Y. (1993) Toxicon 31:1257–1265 describe analgesic effects evoked by low molecular weight substances extracted from Crotalus durissus terrificus venom by ultrafiltration. The extract was administered to mice subcutaneously, intraperitoneally and orally.
CN 1,072,344 published May 26, 1993 discloses a snake toxin ointment containing a commercial snake toxin enzyme (source not given), a leukocyte peptide factor and Bingpian, a known Chinese analgesic medicine. The ointment functions as an antibiotic with no toxicity or side effects.
Pu, X. C., Wong, P. T. H. and Gopalakrishnakone, P. (1995) Toxicon 33:1425–1431 describes a neurotoxin purified from king cobra venom by gel filtration and HPLC. The toxin was administered i.p., p.o. or i.c.v. to mice and found to have a potent analgesic effect.
U.S.S.R. Patent No. 435,824 describes an analgesic composition prepared from Nayaksin dry cobra venom. This snake is from the Naja species which belongs to the Elapidae family.
For over 20 years, an ointment named Viprosalum or Viprosal has been available in the former Soviet Union and in Eastern Europe for the relief of pain. This ointment is a mixture of a viper venom (European species) dissolved in Vaseline together with Lanolin, camphor and solicilate.